Or recall “Cool Colors are Kewl,” a tidbit of mine in R&T, February 2009. Or, for the latest update, see “Cool Paint Job Fights Solar Warmth”, by Robert F. Service, in Science, September 28, 2018, published by the American Association for the Advancement of Science.

Researchers at the University of Colorado in Boulder have formed a startup company, Radi-Cool, Inc., to market what they call “a metamaterial that enhances natural cooling without power input.”

Radi-Cool’s Xiaobo Yin, left, and Ronggui Yang hold a roll of the company’s passive radiative cooling film. This and the following image from Radi-Cool, Inc.

As described at Radi-Cool’s website, “Radiative cooling is a natural process through which objects shed heat in the form of infrared (IR) radiation. All materials at room temperature emit infrared at wavelengths of 5–15 μm. [The micrometer, 1 μm, is 1000th of a millimeter, sort of half way to the nanoworld. There are 1000 nanometers in one micrometer.] However, the process is typically not very efficient because it is counteracted by external influences that heat the object, such as sunlight and air currents. Air, meanwhile, absorbs and emits very little radiation with wavelengths 8–13 μm. The Earth cools itself at night by emitting infrared through this ‘atmospheric window’ into space.”

Radi-Cool’s technology uses glass spheres, ~10 μm in diameter, in a flexible polymer to form a scalable, thin-film cooling material.

According to Robert F. Service in Science, “The new materials, Yin says, could drop cooling cost by up to 15 percent in some climates…. And with 17 percent of all residential electricity use in the United States going toward air conditioning the savings could be substantial.”

Service continues, “White paints typically reflect only about 80 percent of visible light, and they still absorb ultraviolet (UV) and near-infrared (near-IR) rays, which warm buildings.”

Previous efforts in enhanced reflectivity have been developed for new construction. “But,” Service notes, “it’s been harder to come up with options for existing buildings.”

Yuan Yang, Nanfang Yu, Jyotirmoy Mandal, and research colleagues at Columbia University and Argonne National Laboratory have developed a polymer to mitigate this shortcoming. Service describes the technology: “The polymer starts as a solution in acetone, to which the researchers add a small amount of water. When painted on a surface, the acetone quickly evaporates, and the polymer separates from the water, creating a network of water droplets. Finally, the water also evaporates, leaving a sponge-like arrangement of interconnected voids that reflect up to 99.6 percent of light, including IR, visible, and UV.”

Service continues, “Under the relentless midday sun of Phoenix, painted surfaces remained 6 degrees Celsius [approximately 10 degrees Fahrenheit] cooler than the surrounding air, the researchers report in a paper published online in Science this week. And for good measure, they also showed that they could dye the paint, varying its appearance, although the colored paint sacrifices some cooling.”